1. Field of the Invention
The present invention relates to exhaust systems for internal combustion engines and, particularly, to an exhaust system having an exhaust chamber.
2. Description of the Related Art
Exhaust systems for engines, such as two stroke engines installed in vehicles such as personal watercraft, have exhaust chambers that are carefully tuned or shaped to a particular configuration so as to produce optimal engine performance. The purpose of this tuning is to cause exhaust pulse sound waves to reflect inside the exhaust chamber thereby creating local pressure variations. The pressure variations either help scavenge the remaining exhaust gases or plug the exhaust port to prevent the intake charge from exiting into the exhaust.
Exhaust chambers in typical systems are tuned for a particular RPM level. However, the more precisely the system is tuned for a particular RPM level, the more performance throughout the rest of the RPM range will suffer. The speed of the exhaust pulse wave changes according to the formula v=(xkT).sup.1/2, where x is the ratio of specific heat at constant pressure to specific heat of constant volume, k is the gas constant per unit mass and T is the temperature in Kelvins.
As RPM level increases, temperature increases. Since high speed performance is generally emphasized, the exhaust chamber is designed to be relatively short in order to reflect the high speed sound wave at the elevated exhaust temperatures of high RPM engine operation. Such a design, however, adversely affects performance at lower RPM levels, where the reflected exhaust pulse wave reaches the engine exhaust port too early to optimally seal the engine exhaust port during the compression stroke.
Accordingly, there is a need for an exhaust system which can be tuned to produce maximum power at a plurality of an engine's RPM levels. There is more particularly a need for a system which optimizes engine power at relatively lower RPM levels. There is a still further need for a system in which an exhaust pulse wave reaches the engine exhaust port at a time adequate to effectively seal the engine exhaust port during the compression stroke at lower RPM levels.